This question may have been considered before...Has anyone
considered the
idea of using a rotary engine with a speed increaser to drive a
low-pressure
fan similar to one in front of a high bypass turbofan engine? Simply
put,
the engine would run at aprox. 6000 rpm and the fan would turn at 10,000
RPM or so. Then the airflow would be necked down via a funnel output to
create a vectored thrust.Any past articles, threads or comments on this
one?Thanks!

Kevin in Denver

That is called a ducted fan.

Perry Mick experimented with that for about ten years
as I recall.

http://mickaircraft.com/duckt.htm

A half a dozens medium speed ducted fan airplanes have been built
by others and they all failed.

It is not until you get over 400 MPH that ducted fans become
more efficient than a un ducted propeller. That too is now being
challenged with new high speed propeller designs.

Putting it another way.... if you have enough HP to push the airplane
faster than 400 MPH than by all means use a ducted fan.

Lets say you get the drag down to the equivalent of a two square
foot flat plate. The dynamic pressure at 400 MPH is 416 pounds
per square foot. So the drag is 832 pounds.

400 MPH is 587 feet per second. The HP required is therefore
587 X 832 or 488,000 pound feet per second.
One HP is 550 pound feet per second.

Therefor you will need 887 net HP out of the duct. You might be
able to pull it off with a 1200 HP turbo charged 3 or 4 rotor Wankel
engine just to go 400 MPH. If you wanted to go 450 MPH you would
need a lot more HP as top speed is proportional to HP cubed.
With a 1200 HP rotary engine the fuel burn would be about
600 pounds per hour or about 100 gallons an hour.
A turbo compound version would do 20% better.

A Cessna Citation jet that flies about the same speed
(352 Kts) burns about 140 gallons an hour.
http://www.jetadvisors.com/aircraft-specs/Description-CessnaCitationCJ2-SmallCabinLightJets.htm

Airliners also use ducted fans but modern high bypass turbo jet engines
are in the range of 50,000 HP and up.

Paul Lamar

Several points.  vtol is relatively easy to achieve, it is the
translation to forward flight that kills you.  The speed range in
which the vehicle must operate is also critical.  Form follows
function.  What does this system have to do?  Are you working on a
low cal Harrier or?  You might want to consult Chapter 9 of
Aerodynamics of V/STOL Flight by Barnes W.  McCormick, Jr.  Yes,
some of us have looked at rotary based systems using both PSRU and
speed increasers.  They add complexity, maintenance issues and
weight.

The majority of "ducted fans" used in VTOL were actually
shrouded props.  They had low solidity, large diameters, relatively
low loadings and substantial weight.  You can get into fist fights
over defining just what constitutes a ducted fan, but most who have
worked on them pretty much agree on solidities over 1, using cascade
theory because of the overlaps, ducts with a chord of at least one
rotor diameter and pressure ratios well above the shrouded prop
variety.  For a 250 mph aircraft the pressure ratio would be about
1.06.

Shrouded propellers frequently work in the 1.01-1.03 range.
If you are looking for a wide range of speed, a variable nozzle
becomes an issue.  If you are working on a fixed fan geometry, then
the aerodynamics of your vectored thrust system and the losses
entailed become the big problem.  Ask Boeing about their Harrier
derived version of the VTOL strike fighter competition.  The bottom
line question is:  are my basic assumptions about the need for
VTOL/vectored thrust valid or can I achieve a valid outcome without
it?  If you're doing it because it seems cool and can afford it, go
for it.  If you are doing it for a commercial reason be sure the
market is actually there.  Engineering is full of solutions looking
for a problem.

best,

George W